The problem exists of how to realize a powerful sensor that can determine various properties in liquids and gases, such as the presence of an analyte, the concentration of an analyte in a liquid or gas, the affinity and energetics of chemical bindings, and even the precise flow rate in microchannels. A further problem includes a means to weigh biomolecules, single cells and nanoparticles in fluids.
In any resonant system, such as a microresonator, the quality factor (Q) is of main interest. It is defined by the ratio of the vibrational energy stored in the system and the total energy lost per cycle. The higher Q, the better the frequency stability of the resonator, which translates to lower phase noise performance, the main parameter for lowest detection limits of mass-loading-based sensors. A microresonator for mass loading applications has to have contact to the gas or liquid of interest. This inevitably introduces a main damping source, and, thus, significantly compromises the quality factor. A good example of this dilemma is the design of nano-cantilevers for gas sensing applications. In order to maximize for high quality factors (up to 400), these nano-cantilevers were designed down to sizes of the mean free path at atmospheric pressure, which counteracts the main purpose of such sensor, i.e. to interact with the gas of interest in terms of mass loading to sense something. Another demonstrated solution, although impractical, is to operate the microresonator at reduced pressure to avoid the quality factor degrading viscous damping losses present at atmospheric pressure.
In general, a cantilever-based approach suffers from the fact that the moving object is completely surrounded by the medium, which results in higher damping due to higher viscous drag and acoustic pressure waves to more than one side.
For mass loading applications in gases, the approach of using CMUT-based instead of cantilever-based microresonators is successful, but in liquids even a CMUT-based microresonator is heavily damped due to the high acoustic energy loss into the liquid, mainly due to the higher density than compared to a gas. It has been shown that the achievable quality factors of CMUTs immersed in liquids is too low to be functional as mass-loading sensor. Theoretically the manipulation of the boundary condition inside a CMUT element, comprising several CMUT cells, from rigid baffle to pressure-released baffle should increase the quality factor Q significantly, but for this approach to work each cell must be surrounded by a non-active area, i.e. an area that behaves closely to a pressure-released baffle. Another approach considered was to actively actuate neighbouring cells with an out-of phase signal to increase the quality factor, but the improvement in terms of quality factor was marginal.
There are existing approaches for cantilever-based microresonators to overcome the problem of low Qs when immersed in liquids. Q-enhancement technique has been described that decreases the effective damping of the cantilever by using an external feedback amplifier and a phase shifter (in a feedback loop), which allows increasing the quality factor by more than one order of magnitude. The method allowed increasing the very low quality factor of a microfabricated cantilever from ˜1 to ˜31. However, simulations have shown that such Q-enhanced systems are highly non-linear and the external amplifier can start to dominate the system. Further, a quality factor of 31 is still low compared to values achieved in air (up to 400). The most promising approach for cantilever-based biosensors is to embed the microchannel inside the cantilever itself. The main idea is to confine the fluid of interest inside the resonant cantilever while leaving the cantilever itself in a gaseous environment, or vacuum.
What is needed is a device that measures at least one property of the liquid or gas, which can be related to the presence of a chemical species of interest, or to measure at least one property of one or several objects inside the liquid or gas.